Scalable pt cluster and ruo2 heterojunction anode catalysts

ABSTRACT

A synthesis process for forming nanodendrites. The nanodendrites are utilized in a process to form a heterojunction catalyst. Nanodendrites may include PtRu8 nanodendrites that can be oxidized through annealing to form PtRuO2. One heterojunction catalyst comprises PtRuO2 on a carbon support.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.16/584,385, filed Sep. 26, 2019, the contents of which is incorporatedherein by reference in its entirety.

STATEMENT OF GOVERNMENT INTEREST

This invention was made with government support under Contract No.DE-AC02-06CH11357 awarded by the United States Department of Energy toUChicago Argonne, LLC, operator of Argonne National Laboratory. Thegovernment has certain rights in the invention.

TECHNICAL FIELD

The present disclosure relates to catalytic materials, specifically toheterojunction materials from nanodendrites.

BACKGROUND

The recent development of novel membranes and ionomers with enhancedhydroxide conductivity and alkaline stability opens up new opportunitiesfor the alkaline anion-exchange membrane fuel cells (“AEMFCs”). However,most of the membrane electrode assemblies (“MEAs”) demonstrating highpeak power density in literature used a substantial amount of Pt-groupmetal (“PGM”) catalysts for the anode and cathode. Developingcost-effective oxygen reduction reaction (“ORR”) catalysts for AEMFCshas been partly successful as the MEAs employing less expensive ORRcatalysts showed ˜1 W/cm² peak power density. However, developinglow-cost electrocatalysts for the hydrogen oxidation reaction (“HOR”) ofthe AEMFCs has limited success; therefore, the cost-benefits of AEMFCsover the proton-exchange membrane fuel cells (“PEMFCs”) have diminished.Replacing Pt-based HOR catalysts with highly active Pd-based catalystshas become less attractive as the price of Pd is getting higher thanthat of Pt. Replacing Pt catalysts with non-PGM catalysts has had onlylimited success because of the relatively low catalytic activity andanode flooding. Reducing Pt loading in the AEMFC anode is a potentialsolution to reduce the overall cost of AEMFCs. Recently, Omasta, et al.,reported ˜0.8 W/cm² peak power density with a low Pt-loadinganode-catalyzed MEA (Anode Pt loading=0.073 mg_(Pt) cm⁻²).

To develop highly efficient HOR catalysts for the advanced AEMFC system,not only does the intrinsic kinetic activity of the electrocatalyst needto be taken into account, so too do other parameters, such ascompatibility of the catalyst with ionomer of MEA, water management, andthe scalability of the catalyst synthetic procedure. Our previous workof HOR on bulk alloy electrode surfaces demonstrated that Pt—Rubimetallic alloy catalysts have benefits because Ru would provide thesites for OH_(ad), which can then effectively remove the hydrogenintermediates that are present on the nearby Pt sites. Further, work atLos Alamos National Laboratory found that Pt—Ru bimetallic alloycatalysts also have benefits to minimize the phenyl group adsorptionwhich significantly increases the HOR current density up to 0.5 V vs.reversible hydrogen electrode (“RHE”). Since most ionomeric binders usedfor the electrolytes of AEMFCs contain phenyl group, the unique lowphenyl group adsorbing characteristics of Pt—Ru bimetallic alloycatalysts have shown the dramatic increase in AEMFC power density to˜1.5 W/cm² for MEAs using polyaromatic ionomers. Other researchersdemonstrated the AEMFC power density of >2.0 W/cm² with the MEAs usingless phenyl-containing polyolefinic ionomers. Incorporating Ru elementto Pt catalyst may significantly reduce the HOR catalyst cost as the Ruprice is only ˜30% of Pt; however, it is still challenging to maintainthe excellent anode performance at low Pt-loading anode with Pt—Rubimetallic alloys because the water generated from HOR in the thincatalyst layer promotes anode flooding.

SUMMARY

At least one embodiment relates to a method of forming a PtRuO₂heterojunction catalyst. The method includes forming a solution ofplatinum precursor, a ruthenium precursor, diphenyl ether,1,2-tetradecanediol, oleylamine, and dichlorobenzene. The solution isheated to a reaction temperature between 230-270° C. for 5 min-1 h,forming nanodendrites. The nanodendrites are separated from the solutionand suspended in organic solvent forming a suspension. The carbon ismixed with the suspension. The carbon material is isolated from thesuspension and annealed, converting Ru to RuO₂.

Another embodiment relates to a method of forming a nanodendrites. Themethod comprises forming a solution of platinum precursor, a rutheniumprecursor, diphenyl ether, 1,2-tetradecanediol, oleylamine, anddichlorobenzene. The solution is heated to a reaction temperaturebetween 230-270° C. for 5 min-1 h. PtRu₈ are separated nanodendritesfrom the solution.

Yet another embodiment relates to a heterojunction catalyst. Thecatalyst comprises a carbon substrate and a catalytic material supporton the substrate, the catalytic material comprising PtRuO₂. The Ptparticles have a diameter of 1-5 nm and RuO₂ particles have a diameterof 2-20 nm, the Pt particles and the RuO₂ particles being atomicallyconnected.

This summary is illustrative only and is not intended to be in any waylimiting. Other aspects, inventive features, and advantages of thedevices or processes described herein will become apparent in thedetailed description set forth herein, taken in conjunction with theaccompanying figures, wherein like reference numerals refer to likeelements.

BRIEF DESCRIPTION OF THE FIGURES

FIGS. 1A-1H show transmission electron microscopy (“TEM”)characterization of PtRu₈ nanodendrites: TEM image of as synthesizednanodendrites (FIG. 1A), TEM image (FIG. 1B), high-resolutiontransmission electron microscopy (“HRTEM”) image (FIG. 1C), select areaelectron diffraction (FIG. 1D), high-angle annular dark-field (“HAADF”)image (FIG. 1E), and energy-dispersive X-ray spectroscopy (“EDS”)mapping of carbon-supported PtRu₈ nanodendrites (FIGS. 1F-1H).

FIGS. 2A-2H show TEM characterization of Pt—RuO₂ heterojunctions: TEMimage (FIG. 2A), HRTEM image (FIG. 2B), select area electron diffraction(FIGS. 2C-2D), HAADF image (FIG. 2E), and EDS mapping of Pt—RuO₂heterojunctions (FIGS. 2F-2H).

FIGS. 3A-3B show cyclic voltammograms (FIG. 3A) and HOR curves (FIG. 3B)of Pt—RuO₂ and Pt TKK in 0.1 M sodium hydroxide (“NaOH”). FIGS. 3C-3Dshow cyclic voltammograms (FIG. 3C) and HOR curves (FIG. 3D) of Pt—RuO₂and Pt TKK in 0.1 M benzyltrimethylammonium hydroxide (“BTMAOH”). Cyclicvoltammograms were recorded at 50 mV/s; HOR polarization curves wererecorded at 20 mV/s, 900 rpm.

FIG. 4A shows AEMFC performance comparison between MEAs; anode catalyst:Pt—RuO₂/C, TKK-Pt/C, and JM HiSPEC® 12100 Pt—Ru/C; cathode catalyst:Pt/C (0.6 mg_(Pt)/cm²). AEMFC performance at 80° C. with humidified H₂(2000 sccm) and O₂ (1000 sccm) at 285 kPa backpressure. FIG. 4B showsAEMFC peak power density comparison as a function of anode catalystcost; AEMFC performance was taken from literatures. Anode catalyst costwas calculated from a 5-year average of base metal price.

FIGS. 5A-5B show TEM images of scale-up Pt—RuO₂ nanoparticles supportedon high surface area carbon; FIG. 5C shows the corresponding X-raydiffraction (“XRD”) pattern (RuO₂ standard PDF#01-071-48251; Pt standardPDF#01-087-0647).

FIG. 6 shows a graph of the kinetic performance of Pt/C and Pt—RuO₂/Canode catalyzed MEAs in terms of iR-corrected polarization curves at 80°C. with humidified H₂ (2000 sccm) and O₂ (1000 sccm) at 285 kPabackpressure. Cathode: Pt/C (0.6 mg_(Pt)/cm²).

FIG. 7 is a graph of the effect of Pt loading at anode on AEMFCperformance. Anode catalyst: JM HiSPEC® 12100 PtRu/C, cathode: JMHiSPEC® 9100 Pt/C (0.6 mg_(Pt)/cm²). Fuel cells performances wereobtained at 80° C. with fully humidified 2000 sccm H₂ and 1000 sccm O₂at 285 kPa backpressure.

FIGS. 8A-8B are graphs showing the effect of flow rate on AEMFCperformance. FIG. 8A shows Pt—RuO₂/C anode catalyzed MEA, and FIG. 8Bshows JM HiSPEC® 12100 Pt—Ru/C anode (0.5 mg_(Pt)/cm²) catalyzed MEA at80° C. and 285 kPa backpressure under fully humidified gas feed.

FIG. 9 is a graph of the AEMFC performance in H₂/CO₂-free air withPt—RuO₂/C as an anode and Pt/C as a cathode. Fuel cells performanceswere obtained with fully humidified 2000 sccm H₂ and 1000 sccm CO₂-freeair at 285 kPa backpressure.

FIGS. 10A-10C show TEM images and EDS spectroscopy of PtRu nanoparticlessynthesized in dibenzyl ether with oleylamine/oleic acid (0.5 mL each)as surfactants and 1,2-tetradecanediol (84 mg) as reducing agent.

FIGS. 11A-11C show TEM images and EDX spectroscopy of PtRu nanoparticlessynthesized in diphenyl ether with one-pot synthesis.

DETAILED DESCRIPTION

Before turning to the figures, which illustrate certain exemplaryembodiments in detail, it should be understood that the presentdisclosure is not limited to the details or methodology set forth in thedescription or illustrated in the figures. It should also be understoodthat the terminology used herein is for the purpose of description onlyand should not be regarded as limiting.

Certain embodiments relate to a Pt—RuO₂ heterojunction catalysts. Suchcatalysts may be used for cost-effective HOR. Rather than strictlyalloying elemental Pt and elemental Ru, one embodiment synthesizesPt—RuO₂ heterojunction catalysts from PtRu₈ nanodendrites.

One embodiment relates to a method of forming heterojunction catalysts.Specifically, heterojunctions may comprise Pt—RuO₂ catalysts or areother stoichiometric ratios. Further, other elements, such as Ni and Co,may be used rather than Ru.

The heterojunction catalysts are prepared from nanodendrites; oneembodiment uses PtRu₈ nanodendrites. In one embodiment, the PtRu₈nanodendrites are formed through a solvothermal synthesis process usingan organic solvent, such as diphenyl ether, and an organic reducingagent, such as an organic diol (e.g., 1,2-tetradecanediol), and asurfactant, such as oleylamine, which helps the particle uniformity andsize control.

The metals that form the nanodendrites are supplied via respectiveprecursors, for example platinum precursors and ruthenium precursors Inone embodiment, the Pt precursor is platinum (II) acetylacetonate[Pt(acac)₂]. In one embodiment, the Ru precursor is ruthenium(III)acetylacetonate [Ru(acac)₃]. The ratio of Pt precursor to Ru precursorshould be selected to approximate the desired ration in the catalyticmaterial, such as for PtRu₈, a ratio of Pt:Ru precursors of 1:12 to 1:6.

In one embodiment, the nanodendrites are formed through addition of 5-15mg/ml (e.g., 10 mg/ml) Ru(acac)₃, 0.5-1.5 mg/ml (e.g., 1 mg/ml)Pt(acac)₂ in diphenyl ether with 0.2-10 mg/ml (e.g., 5 mg/ml) of1,2-tetradecanediol and 0.01-0.5 ml/ml (e.g., 0.2 ml/ml) oleylamine.These relative amounts may be scaled in some embodiments for a largerreaction batch. The nanodendrite reaction proceeds at a dendritereaction temperature, such as 230-270° C. (e.g., 260° C.), and adendrite reaction time, such as 5 min to 1 h. In one embodiment,dichlorobenzene is not injected until the temperature of the solution of1,2-tetradecanediol and diphenyl reaches 200° C. as it is being heated.In another embodiment, a one pot synthesis approach is utilized with allof the ingredients added and the solution heated to the dendritereaction temperature.

PtRu₈ nanodendrites are separated from the solvents. In one embodiment,the separation is by centrifuge (10000 rpm for 10 min). The collectednanodendrites are then dispersed in an organic solvent, such aschloroform.

The collected nanodendrites are then used to form the heterojunctioncatalyst. The nanodendrites suspended in an organic solvent are mixedwith carbon. In one embodiment, the carbon is in the form of carbonnanoparticles, such as 20-100 nm in diameter. In one embodiment, theratio of carbon to catalytic material (such as PtRuO₂) nanodendrites is50:1 to 3:1. The mixture is agitated, such as by sonication, for amixing time and then precipitated, such as by addition of hexane.

The nanodendrite/carbon mixture is annealed to convert the elemental Ruto RuO₂. The annealing step also removes remaining solvent orsurfactant. In on embodiment, the annealing is at an annealingtemperature within the range of 100-200° C. (e.g., 185° C.) in anambient air environment. In one embodiment, the annealing is for anannealing time, such as overnight (8-12 hours).

In one embodiment, the heterojunction catalyst comprises ultrafine Ptparticles atomically connected with RuO₂. Ultrafine particles are thosebelow 5 nm, such as 1-5 nm, preferably 2-3 nm. RuO₂ size is not welldefined and can be 3-5 nm to 20 nm. In one embodiment, 2-3 nm (orsmaller) Pt is surrounded by 2-3 nm (or smaller) RuO₂. The structure ofthe catalytic material is a heterojunction with atomically connectedinterface between Pt and RuO₂.

Certain embodiments may facilitate a lower electrode loading due to theimproved performance properties.

Experimental Results.

Carbon-supported heterojunction catalysts were prepared by solvothermalsynthesis and subsequent thermal treatment. Scale-up synthesis to 1g/batch yield enables a thorough investigation of its MEA performance incombination with rotating disk electrode (“RDE”) studies. The resultsshow a unique morphology with ultrafine Pt particle sizes, andatomically connected interfaces between Pt and RuO₂ provide highcatalytic activity toward HOR while maintaining significantly improvedH₂ mass transport in MEAs via minimizing undesirable phenyl groupadsorption in the polymer electrolyte. The results also demonstrate MEAperformance of low anode loading Pt—RuO₂/C with those of thestate-of-the-art Pt/C and Pt—Ru/C catalysts.

In summary, Pt—RuO₂ heterojunction catalyst with ultrafine Pt clusterand atomically connected interface was developed by converting Ru-richphase of PtRu₈ nano-dendrite into Pt—RuO₂. The synthetic condition ofPtRu₈ nanodendrites were investigated and preliminary scale-up wasexplored. With successful demonstration of one-pot synthesis, furtherscale-up should be attainable. The Pt—RuO₂ heterojunction catalystshowed excellent catalytic activity towards HOR and significantly lowerphenyl group adsorption properties compared with commercial Pt/Ccatalyst. The AEMFC test suggests that the structure of the Pt—RuO₂heterojunction catalyst provides high access of H₂ at ultra-low loadinganode in combination of the good kinetic activity and less degree ofphenyl adsorption, making an ideal low PGM loading catalyst for AEMFCs.This result is the first report that that highly active Pt—Ru bimetallicHOR catalyst can be prepared without alloying Pt and Ru components butproviding unique morphology of ultrafine Pt cluster and Pt—RuO₂heterojunctions.

Synthesis of Pt—RuO₂ Heterojunction Catalysts.

The Pt—RuO₂ heterojunction catalyst was prepared from PtRunanodendrites. Several pathways to the PtRu nanoparticles with highsurface area were explored using different combinations of solvent,surfactant, and reducing agent. Some of the pathways were seen toproduce Pt rich PtRu nanoparticles, while others produced Ru rich PtRunanoparticles, which are the desired form for the heterojunctioncatalyst.

Dibenzyl Ether.

As shown in FIG. 10A, we found Pt₂Ru₁ nanoparticles of ˜5 nm could bemade in dibenzyl ether solvent at elevated temperature (FIGS. 10A-C).Specifically, this pathway used dibenzyl ether with oleylamine/oleicacid (0.5 mL each) as surfactants and 1,2-tetradecanediol (84 mg) asreducing agent. The particles synthesized in dibenzyl ether in thepresence of oleic acid is Pt-rich and not uniform because the presenceof oleic acid hinders the reduction of Ru precursors; thus, we did notpursue this synthetic route.

Further, a sample with 1,2-tetradecanediol as a reducing agent resultedin an elemental percentage of PtL 69.7 atomic %, RuK 31.3 atomic %.Without reducing agent, Ru content in the particle is even lower (PtL85.4 atomic %, RuK 14.6 atomic %), indicating the mild reducingcapability of oleylamine is not sufficient to completely reduce Ruprecursor under this reaction condition. Even with this modification,the dibenzyl ether route does not provide sufficient Ru content.

Diphenyl Ether.

Ru-rich nanoparticles with minimal particle size and uniform sizedistributions could be synthesized in diphenyl ether using1,2-tetradecanediol and oleylamine. Specifically, experiments for thesynthesis of PtRu8 nano-dendrite were performed in an Ar flowenvironment in a round bottom flask. Typically, 0.12 g Ru(acac)₃, 2 mLoleylamine, 0.063 g 1,2-tetradecanediol, and 10 mL diphenyl ether wereheated up to 260° C. in a round bottom flask with Ar flow and 0.012 gPt(acac)₂ dispersed in 1 mL dichlorobenzene was injected when thetemperature of former solution reach 200° C. The mixture was heated upslowly to 260° C. The reaction time was controlled to 20-30 min startingfrom injection.

Scale-up synthesis was performed in a bigger round bottom flask withsimilar synthesis procedure and 6 times higher reaction precursors andreaction volume. Note, severe boiling was observed above 230° C. becausethe boiling point of dichlorobenzene is only 180° C. Care should betaken on the heating rate to avoid pressure buildup in the flask.Composition of dichlorobenzene could be decreased for larger reactionvolume.

Further, a one-pot synthesis was performed in a similar procedure exceptthat all the reaction precursors were heated up in a round bottom flask.By doing so, we eliminated the hot injection step.

For those nanodendrites to be used further in formation of the catalyst,the PtRu₈ nanodendrites are separated from the solvents by centrifuge(10000 rpm for 10 min). The collected nanodendrites are then dispersedin chloroform for the annealing step.

Similar particle size and composition were obtained with both 0.5 mL and2 mL oleylamine as a surfactant and/or slightly changed amount ofreducing agent (relative to the prior samples tested) (FIG. 1A) forwhich the lower surfactant and reducing agent amount is sufficient forthe reaction and further increase does not affect the productsignificantly. Samples yielded a content of: 0.5 mL surfactant and 84 mgreducing agent (PtL 16 atomic %, RuK 84 atomic %); 2 mL surfactant and84 mg reducing agent (PtL 14.6 atomic % m RuK 85.4 atomic %); and 2 mLsurfactant and 65 mg reducing agent (PtL 16.5 atomic %, RuK 83.5 atomic%).

The Ru content in the particle is only slightly lower than the precursorratio, indicating a very high conversion rate of the Ru precursor. Asshown in FIG. 1B, PtRu₈ nanodendrites can be loaded onto carbon supportuniformly. HRTEM image (FIG. 1C) and selected area electron diffraction(“SAED”) pattern (FIG. 1D) demonstrated the crystalline nature of thenanodendrites. Although the SAED pattern is weak with ultra-smallparticle size, HAADF imaging and corresponding EDS mapping revealed thatthe PtRu₈ nanodendrites are composed of Pt-rich core surrounded withRu-rich branches (FIGS. 1E-1H). From these results, the formation ofPtRu₈ nanodendrites starts with Pt-rich nuclei because Pt is easier tobe reduced than Ru. As the reaction progresses, Pt(acac)₂ is depleted,and Ru-rich branches continue to grow on the surface, leading to thedendrite structure.

Collected PtRu₈ nanodendrites were dispersed in chloroform and were thenmixed with proper amount of carbon, which can be calculated from thetarget PtRu loading on carbon, which was also dispersed in chloroform bysonication. The mixture was sonicated for 20 min and the carbonsupported PtRu₈ nanodendrites were precipitated with hexane and furtherseparated from solvents by centrifuge. This process also works forscale-up sample preparation; larger amounts of solvents were used toensure good dispersion of PtRu₈ nanodendrites on carbon. The carbonsupported PtRu₈ nanodendrites were annealed in air at 185° C. overnightto convert elemental Ru into RuO₂ and to remove the surfactant adsorbedon the surface of catalyst.

Initially synthesis the Pt—RuO₂ heterojunction catalysts on a smallscale (0.2 g/batch). Later, a scale-up synthesis of the Pt—RuO₂heterojunction catalyst was investigated by addressing heat transfer andmass transport challenges. With six times higher volume reaction, theboiling of the reactants above 230° C. is more severe than small batchsynthesis because the amount of dichlorobenzene (boiling point=180° C.)is also six times higher while the removal rate of dichlorobenzene byargon flow is limited. The heating rate also decreases as more time isneeded for reactants to reach the same reaction temperature. However,similar particle size and composition was obtained with the scale-upsynthesis (content 14.1 PtL atomic % and 85.9 RuK atomic %). Moreimportantly, scale-up synthesis is highly reproducible.

The process of loading PtRu₈ nanodendrites on carbon is also scalable.Overall, more than 1 g of Pt—RuO₂/C heterojunction catalyst was obtainedby combining the two batch of scale-up synthesis and subsequentannealing in air. Notably, injecting precursors into hot reactivesolution (hot-injection) is not favorable for scale-up. Similar particlesize and composition were obtained with one-pot synthesis, indicatingthis newly developed recipe is indeed scalable.

The Pt—RuO₂ heterojunction catalyst was obtained by annealing of thecarbon supported PtRu₈ nanodendrites in the air at 185° C. overnight,and Ru was converted into RuO₂. As shown in FIGS. 2A-2H, the uniformdendrite structure was converted into a composite structure with lowercontrast due to RuO₂ formation. The HRTEM image in FIG. 2B and SEAD inFIGS. 2C-2D indicate the formation of Pt—RuO₂ heterojunction structureswith atomically connected interfaces. The HAADF imaging and EDS mappingresults in FIGS. 2E-2H further demonstrate the Pt—RuO₂ heterojunctionstructure. Compared with the PtRu₈ nanodendrites, the size of thePt-rich particle is slightly smaller, indicating some of the Ru in thesePt-rich areas is also oxidized and migrated to the RuO₂ region. Alsoobserved was some Pt dispersed in the Ru-rich area, which may bebeneficial to the performance at a low-Pt loading AEMFC anode. In someembodiments, during the formation of Ru rich dendrites, there are stillPt precursors left, thus small amount of Pt is observed in Ru richdendrites.

ICP-MS confirms that the atomic ratio of Pt:Ru in the heterojunctioncatalysts prepared from the scale-up synthesis is 1:8. TEM images (FIGS.5A-5B) showed the six times scale-up Pt—RuO₂ nanoparticles aremonodisperse (most particles fall in the range of 3-6 nm in diameter)and are uniformly distributed on high surface area carbon support. XRDpattern also proves that the RuO₂ dominates the particle composition(FIG. 5C). The carbon supported Pt—RuO₂ heterojunction catalyst from thescale-up synthesis was further investigated in the RDE and AEMFCtesting.

Electrochemical Characterization of Pt—RuO₂ Heterojunction Catalysts.

While RDE test in the acidic electrolyte (mostly perchloric acid andsulfuric acid) has been proved as an efficient technique for thecatalyst screening of PENIFCs, a systematic study to compare the RDEresults in the alkaline electrolyte with the AEMFC performance is yet tobe established. Conventionally, NaOH or KOH electrolyte has been used toreveal the kinetic performance of the catalyst in an RDE setup. Ourrecent work suggested that this might be insufficient for catalystscreening of AEMFCs, as catalyst/ionomer interactions such as phenylgroup adsorption and cation-hydroxide-water co-adsorption could mask theintrinsic kinetic performance of a catalyst. In short, under AEMFCoperating conditions, the HOR rate imposes limits on its performance,not to discount the need for continuous improvement of ORR catalysis,which is believed to be facile compared with the sluggish ORR in PENIFC.

In this work, we carried out the RDE study using two differentelectrolytes, namely NaOH and BTMAOH, all in 0.1 M solution of Milli-Qwater. We first compared the HOR performance by the slope between 0 and0.1 mA/cm² instead of the exchange current density, which is difficultto obtain for nanoscale system. Then, we compared HOR current density at0.05-0.3 V, which is relevant to the AEMFC operating condition. Theresults are presented in FIGS. 3A-3D. In NaOH, the TKK Pt/C catalystshows a typical Pt H_(upd) feature between 0.05 V and 0.40 V. Pt—RuO₂catalyst shows big capacitance feature in the same potential region,indicating a RuO₂ rich surface (FIG. 3A). For HOR performance, Pt/C TKKand Pt—RuO₂/C show almost identical kinetic performance near 0 V (FIG.3B, inset). Note that with the same metal loading, the synthesizedPt—RuO₂/C catalyst has approximately 3.5% wt. of Pt only, as opposed tothe commercial catalyst with 19.4% wt. of Pt. The Pt clusters and theirunique heterojunctions with RuO₂ indeed exhibit excellent kineticperformance. When the electrolyte was switched to BTMAOH, TKK Pt/C showsa significant reducing current below 0.2 V, which is likely caused byphenyl group adsorption and subsequent reduction. For Pt—RuO₂, verylittle current drop is observed, which indicates Pt—RuO₂ interactionwith the phenyl group is much weaker than that of pure Pt (FIG. 3C). In0.1 M BTMAOH electrolytes, Pt—RuO₂ shows higher current at 0.05-0.3 V,where the catalyst/phenyl group interactions take place (FIG. 3D).Overall, RDE results suggest that our Pt—RuO₂/C catalyst from thescale-up synthesis has comparable HOR kinetic performance with thecommercial Pt/C but higher tolerance of phenyl group poisoning even atsignificantly lower Pt loading.

Performance of Pt—RuO₂ Heterojunction Catalysts in MEA.

We evaluated the performance of Pt—RuO₂/C heterojunction and otherstate-of-the-art Pt-based anode catalysts in MEA. The MEAs tested havethe same MEA components except for the anode catalyst. The ionomer tocarbon (“I/C”) ratio of the anode was optimized for the best AEMFCperformance. FIG. 4A compares the polarization curve, cell highfrequency resistance (“HFR”) and power density of the MEAs employing lowPt loading Pt/C, PtRu/C, and Pt—RuO₂/C anode catalysts. Under the H₂/O₂conditions, the MEA using Pt—RuO₂/C anode catalyst exhibitsexceptionally high performance in spite of the lower Pt loading. Thepeak power density of the MEA using Pt—RuO₂/C anode reached 0.77 W/cm²,as opposed to the MEA using Pt/C and PtRu/C anode catalysts of which thepeak power density was 0.28 and 0.55 W/cm², respectively. All MEAs havesimilar cell HFR (˜0.045 W/cm²), confirming that the differentperformance is not originated from MEA hydration and catalyst-AEMinterface. The kinetic performance difference between the Pt/C andPt—RuO₂/C catalyzed MEAs is negligible, for example, the iR-correctedcurrent density of the MEAs at 0.95 V are similar (0.026 A/cm² for Pt/Cvs. 0.028 A/cm² for Pt—RuO₂/C) at the similar metal loading (FIG. 6 ),while the performance difference became significant as the currentdensity increases. For instance, the current density of the Pt—RuO₂/Ccatalyzed MEA at 0.85 V is 0.22 A/cm², ˜2 times of the Pt/C catalyzedMEA. This result is consistent with the RDE experiment where the kineticactivity of Pt—RuO₂/C and Pt/C is comparable, but the overall activityof the Pt/C catalyst is hindered by the adverse adsorption of the phenylgroups of the ionomer. The performance comparison between PtRu/C andPt—RuO₂/C catalyzed MEAs indicates that the kinetic performance of theMEA using PtRu/C catalysts is slightly higher than that of the MEA usingPt—RuO₂/C (0.036 A/cm² for PtRu/C vs. 0.028 A/cm² for Pt—RuO₂/C at 0.95V). However, the performance at the high current density region,ca. >1.0 A/cm², the Pt—RuO₂/C catalyzed MEA showed significantly higherperformance than the PtRu/C catalyzed MEA.

The notably higher performance of Pt—RuO₂/C catalyzed MEA could notalone be explained by the phenyl group adsorption since both catalystshave minimal phenyl group adsorption. There are two possible reasonsbehind the high performance of Pt—RuO₂/C. First, the lower ratio ofmetal to carbon in electrocatalysts (15% for Pt—RuO₂/C vs. 75% forPtRu/C) increases the electrode thickness which improves the masstransport at the low loading anode. The electrode thickness effect isalso apparent in the fuel cell performance as a function of anode PGMloading (FIG. 7 ). The peak power density of Pt—RuO₂/C MEA increasedonly about 20% with 4-times higher anode catalyst loading, while thepeak power density of PtRu/C MEA increased ˜3.5 times with 4-timeshigher anode catalyst loading. This indicates that our novel Pt—RuO₂/Cheterojunction catalyst works really well for low PGM loadingelectrodes, on the other hand, the commercial PtRu/C catalysts with highmetal content only perform well at higher PGM loadings. Second, theultrafine Pt clusters surrounded by large RuO₂ particles createdesirable morphology for the mass transport. The superior H2 masstransport of the Pt—RuO₂/C at the low loading electrode is alsoevidenced by the AEMFC performance at reduced H2 flow. The Pt—RuO₂/Ccatalyzed MEA shows only slightly inferior performance of 0.72 W/cm² ata much lower flow rate, while more significant performance loss isobserved for the PtRu/C catalyzed MEA (FIGS. 8A-8B). The Pt—RuO₂/C MEAshows the peak power density of 0.33 W/cm² at 0.48 V, achieving thespecific power (13.2 W/mg_(Pt)) under H₂/CO₂-free air conditions (FIG. 9).

We further compare the reported AEMFC performance as a function of thecost of anode catalyst. For this analysis, we collected the peak powerdensity of AEMFCs using state-of-the-art anode catalysts in literatureand compared the AEMFC performance normalized for anode catalyst costper cm² area based on the 5 year average price of metals. FIG. 4B plotsthe peak power density of the MEAs as a function of anode catalyst costwith two target lines (i.e., 2020 US DOE MEA cost target (0.2 cents/cm²)and the rated performance at rated power (1 W/cm²) under H₂/airconditions. FIG. 4B shows that Pt—Ru alloy catalysts have betterperformance than Pt-, Pd-, Ru-, and Ir-based catalysts at a givencatalyst cost. Among the Pt—Ru alloy catalysts shown, four catalysts arelocated on the upper-performance-cost limit (red dashed line). Thosecatalysts are the commercial PtRu HiSPEC® 10000 (Pt nominally 40 wt. %,Ru nominally 20 wt. %) supported on Vulcan XC—72R carbon. Two Ni-basednon-PGM catalysts are located on or even beyond theupper-performance-cost limit. However, their cell performance needs toimprove to reach the DOE performance target. The Pt—RuO₂ heterojunctioncatalyst we prepared from this work is also located beyond theupper-performance-cost limit, suggesting that the formation of ultrafinePt cluster with Pt—Ru heterojunction is a promising approach to meetfuel cell cost and performance targets for transportation application.Further optimization of catalyst composition and morphology of theheterojunction catalysts with developing non-PGM ORR catalysts may needfor advanced AEMFC systems.

Physical Characterization

The TEM images were obtained on an FEI Tecnai F20 and JEM-2100Fmicroscopes with accelerating voltage at 300 and 200 kV, respectively.Selected area electron diffraction patterns and EDS results wererecorded with JEM-2100F microscope equipped with an Oxford EDS detector.HAADF images and energy dispersive X spectroscopy mapping were recordedon a FEI Talos F200X scanning transmission electron microscope (“STEM”)with an accelerating voltage of 200 kV at the Center for NanoscaleMaterials, Argonne National Laboratory. XRD patterns were recorded froma Siemens diffractometer D5000. The ICP results of PtRu sample are 7.4(Ru):1 (Pt), 7.6:1, and 7.7:1 for 3 runs. It is averaged to 7.6:1 andthe sample in the manuscript was denoted to PtRu₈.

RDE Study.

Synthesized Pt—RuO₂/C (17% metal wt. on high surface area carbon, Pt/Ruatomic ratio 1/6) and commercial Pt/C (TKK TEC10E20A, 19.4% Pt wt.) weredispersed in water under untrasonication. The catalyst concentration ofboth inks was 1.25 mg/mL. The ink was pipetted onto a glassy carbon disk(5 mm in diameter) to make 20 μg/cm² metal loading and dried in air atroom temperature. 10 μL of Nafion D521 (diluted to 0.1% wt.) was thenadded on the surface as a binder to keep the catalyst on the glassycarbon.

A home-made fluorinated ethylene propylene (“FEP”) cell was used forelectrochemical characterization of the catalyst. 0.1 M aqueous solutionof NaOH (99.99% from Sigma Aldrich) and BTMAOH (40% wt. solution inwater from TCI Chemical) were used as electrolyte. A mercury/mercuryoxide (Hg/HgO) electrode (Pine) was the reference electrode and agraphite rod (Sigma Aldrich) was the counter electrode. The referencepotential was converted to RHE. Cyclic voltammograms were recorded innitrogen purged electrolyte. Before hydrogen oxidation reaction curveswere recorded at 900 rpm, the electrolyte was saturated with hydrogenand the working electrode was subject to 1.40 V for 30 seconds to removethe cation adsorption.

Membrane Electrode Assembly.

The catalyst inks for anode were formulated using Pt/C (TKK TEC10E20A,19.4 wt. % Pt), synthesized Pt—RuO₂/C (15% metal loading on TKK carbonsupport) with alkyl ammonium tethered poly(fluorene) (“FLN”) ionomer (5wt. % in 1:1 solution of isopropanol-ethanol) in 20:80 v/v % water —isopropanol solution. The two sets of anodes were prepared withdifferent Pt loading where I/C ratios were 40% for Pt/C and synthesizedPt—RuO₂/C, respectively. The Pt/C (HiSPEC® 9100, Johnson Matthey FuelCells, USA) and FLN ionomer were used for the cathode in all MEAs. Forcathodes, Pt loading and I/C ratio were 0.6 mgPt/cm² and 42%,respectively. The catalyst ink was brush painted on the BC-29 (gasdiffusion layer, 5 cm², 270 μm thickness) on the vacuum table at 60° C.

The quaternized poly(terphenylene) (“TPN”) membrane was used as polymerelectrolyte. Prepared anode, cathode and membrane were used to fabricateMEA after converting to hydroxide form by immersing in 1 M NaOHsolution. The MEA was then placed into the fuel cell hardware (5 cm²,serpentine flow field) supplied by Fuel Cell Technologies Inc.

Single-Cell Tests.

The pure hydrogen at 2000 sccm and oxygen or CO₂-free air at 1000 sccmsupplied to anode and cathode respectively at 100% relative humidity.All the fuel cell tests were performed at operating temperature of 80°C. The polarization curves were acquired at absolute backpressures of285 kPa using a fuel cell station (Fuel Cell Technologies Inc., USA).Built-in impedance analyzer was used to measure the HFR while obtainingthe polarization curves.

Definitions.

No claim element herein is to be construed under the provisions of 35U.S.C. § 112(f), unless the element is expressly recited using thephrase “means for.”

As utilized herein, the terms “approximately,” “about,” “substantially,”and similar terms are intended to have a broad meaning in harmony withthe common and accepted usage by those of ordinary skill in the art towhich the subject matter of this disclosure pertains. It should beunderstood by those of skill in the art who review this disclosure thatthese terms are intended to allow a description of certain featuresdescribed and claimed without restricting the scope of these features tothe precise numerical ranges provided. Accordingly, these terms shouldbe interpreted as indicating that insubstantial or inconsequentialmodifications or alterations of the subject matter described and claimedare considered to be within the scope of the disclosure as recited inthe appended claims.

It should be noted that the term “exemplary” and variations thereof, asused herein to describe various embodiments, are intended to indicatethat such embodiments are possible examples, representations, orillustrations of possible embodiments (and such terms are not intendedto connote that such embodiments are necessarily extraordinary orsuperlative examples).

The term “coupled” and variations thereof, as used herein, means thejoining of two members directly or indirectly to one another. Suchjoining may be stationary (e.g., permanent or fixed) or moveable (e.g.,removable or releasable). Such joining may be achieved with the twomembers coupled directly to each other, with the two members coupled toeach other using a separate intervening member and any additionalintermediate members coupled with one another, or with the two memberscoupled to each other using an intervening member that is integrallyformed as a single unitary body with one of the two members. If“coupled” or variations thereof are modified by an additional term(e.g., directly coupled), the generic definition of “coupled” providedabove is modified by the plain language meaning of the additional term(e.g., “directly coupled” means the joining of two members without anyseparate intervening member), resulting in a narrower definition thanthe generic definition of “coupled” provided above. Such coupling may bemechanical, electrical, or fluidic. For example, circuit A communicably“coupled” to circuit B may signify that the circuit A communicatesdirectly with circuit B (i.e., no intermediary) or communicatesindirectly with circuit B (e.g., through one or more intermediaries).

The term “or,” as used herein, is used in its inclusive sense (and notin its exclusive sense) so that when used to connect a list of elements,the term “or” means one, some, or all of the elements in the list.Conjunctive language such as the phrase “at least one of X, Y, and Z,”unless specifically stated otherwise, is understood to convey that anelement may be either X, Y, Z; X and Y; X and Z; Y and Z; or X, Y, and Z(i.e., any combination of X, Y, and Z). Thus, such conjunctive languageis not generally intended to imply that certain embodiments require atleast one of X, at least one of Y, and at least one of Z to each bepresent, unless otherwise indicated.

References herein to the positions of elements (e.g., “top,” “bottom,”“above,” “below”) are merely used to describe the orientation of variouselements in the FIGURES. It should be noted that the orientation ofvarious elements may differ according to other exemplary embodiments,and that such variations are intended to be encompassed by the presentdisclosure.

Although the figures and description may illustrate a specific order ofmethod steps, the order of such steps may differ from what is depictedand described, unless specified differently above. Also, two or moresteps may be performed concurrently or with partial concurrence, unlessspecified differently above.

What is claimed is:
 1. A heterojunction catalyst comprising: a carbonsubstrate comprising carbon nanoparticles having a diameter of 20-100nm; and a catalytic material supported on the carbon substrate, thecatalytic material comprising PtRuO₂; wherein the Pt particles have adiameter of less than 5 nm and the RuO₂ comprises RuO₂ particles have adiameter of 2-20 nm, and wherein the Pt particles and the RuO₂ particlesare atomically connected.
 2. The heterojunction catalyst of claim 1,wherein the carbon substrate and catalytic material is 50:1 to 3:1. 3.The heterojunction catalyst of claim 1, wherein the Pt particles have adiameter of 1-5 nm.
 4. The heterojunction catalyst of claim 3, whereinthe Pt particles have a diameter of 2-3 nm.
 4. The heterojunctioncatalyst of claim 1, wherein the RuO₂ particles have a diameter of 2-3nm.
 6. The heterojunction catalyst of claim 1, wherein each Pt particleis surrounded by the RuO₂ particles.
 7. The heterojunction catalyst ofclaim 1, wherein the PtRuO₂ is uniformly distributed on the carbonsubstrate.